Typical separation medias include diatomaceous earth, perlite, activated carbon, zeolites and the like, to achieve filtration of adsorption effects. Use of such components in powdered form often generates high pressure drop and/or channeling. Accordingly, powdered filter aids are put into useful forms by a) incorporating them into thin media such as papers, or by depositing them as a thin pre-coat on a substrate to avoid high pressure drop; or b) combining them with a binder so they can subsequently be made into a useful porous structure.
Molded porous filter structures made from the typical separations media could be improved if the amount of binder could be reduced from the usual 20 to 40 percent and if the molded object could be removed from the high pressure compression mold, i.e. if it had high green strength, and subsequently thermally bonded in a self-supporting condition. Porous filter structures can be made by an extrusion process, but such processes are slow and require a large number of extruders for commercial production. Moreover, such processes are limited in the shape of useful filter structures that can be made.
Filter structures can be made via wet or dry processing. An example of the wet process is U.S. Pat. No. 5,443,735 issued to Pall Corporation which relates to the manufacture of so-called immobilized carbon beds. Several methods are disclosed, one of which produces a radial flow pressed block filter by mixing a wet slurry of fibrous materials, activated carbon and an adhesive; pumping the slurry into a mold; pressing out the free water; removing the pressed block from the mold and heating the block to remove moisture; and adding filter materials. Another produces an axial flow filter involving the mixing of carbon particulate and fine powdered polyethylene resin; the mixture being subsequently heated to melt the polymer and join the carbon particles. The invention is based upon the use of fine brass particles to aid in the inhibition of microbial growth in water.
In the dry process, the components are first dry blended, followed by densification and heating of the dry mixture into different forms and shapes. The current state of the dry process art is exemplified by the following patents. U.S. Pat. No. 4,664,683 issued to Pall Corporation teaches the use of molding carbon blocks with particle sizes from about 200 to 2000 microns. The maximum molding pressure is 400 psi. The particle size of the polyethylene powdered binder employed is about 8 to 30 microns. The preferred binding pressure is from 0.3 to 10 psi. The binding temperature is about 50 to 90.degree. F. (28 to 50.degree. C.) above the Vicat softening temperature. These temperatures are equal to or below the melting temperature of the binder. The carbon and binder mixture is heated within the mold and then pressed for 1 to 2 minutes. The cooled carbon block is removed from the mold and found to be self-supporting. The filter blocks have poor physical strength due to the lack of efficient binding between the binder and the carbon particles.
U.S. Pat. No. 4,859,386 issued to Amway Corporation teaches a method for making a molded composite charcoal filter having two shells including the use of ultra-high molecular weight polyethylene as a binder, having a very low melt flow index, on the order of .ltoreq.1 gram/10 minutes, at a level of between 20 and 35 percent by weight. Binders having a high melt flow index are proclaimed as causing blinding of the active sites of the activated carbon. The patent teaches that while the blocks are still in the mold, they are subjected to heat (175 to 205.degree. C.) and pressure (30 to 120 psi) to form a bonded integral composite filter. The drawback of this type of block is weak physical strength due to poor melt flow of the binder. At temperatures just a few degrees above the melting temperature (135 to 138.degree. C.), the block has very poor physical strength. The method relies on raising the temperature to 175 to 205.degree. C. to increase melt flow, which is avoided in the first place, in order to improve binding strength.
U.S. Pat. No. 5,019,311 issued to KT Corporation teaches a similar process, which employs a blend of conventional binders having medium and high melt flow index. These conventional binders are stretched out into a "continuous web matrix" (CWM) by the shearing forces encountered during material blending before extrusion without blocking the active sites, at a temperature substantially above the softening point of the binder. As a result, the required amount of binder can be reduced significantly to about 8 to 20 percent by weight. During the extrusion process, the media is in contact with the barrel and the screw. The extruded media is very hot and soft and must be hardened by rapid cooling to allow for subsequent handling. The typical binder, ethylene-vinyl acetate copolymer (EVA), used in this invention is sheared at a minimum of 145.degree. C., which is at least 30.degree. C. higher than the melting temperature (115.degree. C.). The binder is stretched and spread onto the carbon particles to give good binding strength. However, this process can blind off an excessive number of pores of the carbon. Moreover, the carbon adsorption capacity and efficiency are very sensitive to the temperature variation and shear force of this process.
Thus, while attempts have been made heretofore to manufacture porous structures containing fine particle size filter aids, the art has not provided a facile process by which to combine relatively low amounts of binder resin with fine particles to produce a block having good green strength and favorably low pressure drop during fluid separation or filtration.